An optical recording medium such as a Blu-ray-R disc contains a reflective film inside irrespective of its recording and reproduction methods. This reflective film is generally made of aluminum, gold, silver, alloys thereof, or silicon. Taking a case of, for example, an optical disc such as a CD or a DVD, a thin film made of an alloy of aluminum or gold is used. A pure gold thin film or a pure silicon thin film is used in a semi-transparent film of the DVD. (Refer to Japanese Patent No. 2880190, Japanese Patent No. 2898112, Japanese Patent No. 3365762, and Japanese Patent No. 3655907)
In an optical recording device which uses a blue laser with a wavelength of approximately 400 nm for recording or reproducing information, a gold or silicon thin film does not have sufficient reflectivity as a reflective film of a recording medium thereof because an absorption coefficient for blue light is high. An alloy of silver has sufficient reflectivity, but brings increase in cost.
The optical recording medium such as the CD, the DVD, and the Blu-ray disc is becoming denser and having high capacity, and hence the sophistication of the reflective film of the optical recording medium is required.